1. Field of the Invention
The present invention relates to an ion beam irradiation apparatus for extracting an ion beam from a plasma generated in a chamber through a grid to which a predetermined voltage is applied to emit an ion beam to a predetermined direction. More particularly, the present invention relates, for example, to an ion milling apparatus for processing a surface of an article to be processed using the extracted ion beam, an ion implantation apparatus for implanting ions into a film using the extracted ion beam, an ion beam deposition apparatus for forming a film, and an insulating member used in these apparatuses as a spacer in fixing a grid.
2. Related Background Art
For example, the milling apparatus utilizing an ion beam uses a plasma generated in a chamber as an ion source. The apparatus extracts ions from the ion source using a plurality of grids to which a DC voltage is applied to accelerate the ions in a predetermined direction, and performs milling using the accelerated ions. Generally, each of the grids has a hole portion or a number of hole portions (small holes), and different DC voltages are applied to them in order to optimize the energy of the extracted ions or to optimize the distribution of the ions. Here, the term “milling” refers to a process in which accelerated ions are made to strike a surface of an article to be processed to cause sputtering, thereby achieving a fine processing on the surface (see for example Japanese Patent Application Laid-Open No. 2000-113849).
Between the grids, there is provided insulating spacers, which are generally made of an insulating material, in order to prevent mutual contact of the grids and to keep their intervals constant. When, for example, the milling is carried out using a extracted ion beam, the surface of the article to be processed is sputtered by the ion beam. When the ions are extracted from the chamber through the grids, all of the ions accelerated by the grids do not reach the exterior of the chamber, but a part of the ions impinge on the grids or other parts to cause sputtering. The materials thus sputtered from the surface of the article to be processed or the grids etc. will adhere in some portion in the interior of the apparatus.
If the adhering material is electrically conductive, for example, the insulation performance of the insulating spacers is deteriorated with an increase in the adhering material, which can eventually lead to short-circuit of the grids. In order to avoid such a situation from occurring, it is necessary to perform an operation for removing the adhering film when the material adhering on the insulating spacers develops to some extent or to replace the insulating spacers with new ones. Accordingly, for example in the case that a metal film is to be processed by a milling operation, the insulating performance is fast deteriorated by adhesion of the metal, and therefore it is necessary to perform the operations for preventing short-circuit at a significantly short cycle. As a solution for prolonging the cycle of such operations, Japanese Patent Application Laid-Open No. 2000-301353 discloses an insulating spacer having an enlarged surface area realized by providing an annular projection with a tapered cross section.
The operation of removing the adhering film is generally performed by a blast process or a chemical processing such as acid cleaning. In these operations, when the adhering film is removed, the surface of the insulating spacer itself is subjected to the processing, and the surface will be broken or partly removed. Consequently, the outer shape of the insulating spacer will become small. Therefore, it is considered that the thickness of the insulating spacer will be decreased by the aforementioned processing and the space between the grids becomes small.
The insulating spacer disclosed in Japanese Patent Application Laid-Open No. 2000-301353 is effective in reducing the frequency of the replacement of the spacer. However, the provision of the annular projection results in an increase in the outer diameter of the spacer. Therefore, the size of the annular projection is limited to sizes that do cause interference with the hole portions in the grid. In the above document, the area in which the insulting spacer is used is an area that is spaced apart from the hole portions of the grid. In the ion beam irradiation apparatus, it is necessary to keep the intervals of the plurality of grids substantially constant. Accordingly, it is preferable, if possible, that insulating spacers inserted and fixed between the grids to keep their intervals can be disposed irrespective of whether the hole portions are present or absent.
In the ion beam irradiation apparatus, it is known that the beam irradiation condition changes with time while the apparatus is running. This is because the grid intervals change with time due to heat from the plasma or other factors and the energy of the extracted ions changes as the ion beam irradiation continues in accordance with the grid intervals. Typically, the aforementioned change returns to generally original state, but a small change that cannot be restored will accumulate with an increase in the number of times of running of the apparatus. Thus, the energy distribution of the ions in the ion beam changes with time. Consequently, there is a risk that operations such as milling cannot be performed appropriately unless the condition of the ion beam is minitored continuously to control the energy distribution by feedback.
Generally, ions extracted from a plasma inherits the ion distribution (or the ion density) in the plasma, and therefore the quantity of the extracted ions is not uniform depending on the extracted position. Therefore, when milling of an article to be processed is to be performed, it is required to make the energy or the quantity distribution of the extracted ions uniform. The parameters that can be easily changed in the apparatus include distance between the grids, the distance between the grids and the article to be processed and the diameter of the small holes formed on the grids. By adjusting these parameters, it is possible to make the distribution or the energy of the extracted ions uniform to some extent. In addition, it is considered that the distribution or the energy of the extracted ions can be made uniform by improving the condition of the plasma as the ion source.
However, alteration of the intervals of the grids or the distance between the grids and the article to be processed does not result in improvement in the basic distribution. In addition, the alteration of the plasma condition has not been positively tried so far since it is difficult to be monitored. Accordingly, in order to meet the aforementioned requirement, the method in which the density of small holes formed on the grids in order for ions to pass is varied in accordance with the ion density has been used in many cases, as disclosed for example in Japanese Patent Application Laid-Open NO. 8-129982. Specifically, the number of the small holes in the area of a grid that corresponds to the portion in the plasma in which the ion density is high is made small to reduce the quantity of the ions extracted from this area, and the number of the small holes in the area that corresponds to the portion in the plasma in which the ion density is low is made large so that the quantity of the ion passing through the grid will be made uniform.
To put it differently, in the above-described solution, the portion in the plasma in which the ion density is low is standardized, and in the portion in which the ion density is high, only a part of the ions are extracted so that the quantity of the extracted ion is made uniform. Consequently, there are a large amount of ions that are blocked by the grids and do not contribute to milling or other operations. Thus, an improvement in the operation efficiency of the apparatus is demanded. In addition, the ions blocked by the grids sputter the grid surface, which can be a cause of impurities contained in the ion beam.
In the above-described method, uniformization of the extracted ion quantity is attempted by extracting an appropriate quantity of ions in accordance with the position basically without any improvement in the ion density distribution in the plasma. Therefore, in the case that ion density distribution is extremely inhomogeneous due to some conditions such as the gas type or the discharge pressure, it is considered that there may occur such a situation that the uniformization per se is difficult to be achieved or that the extracted ion quantity of the portion standardized for the uniformization is too small to be practically used. Accordingly, there is the possibility that the extraction of the ion can be effected only in limited conditions, and versatility of the apparatus as such may be restricted.